An optical transmission system is typically composed of: a transmitting terminal transmitting at least one polarized optical carrier wave in which the power or the phase is modulated as a function of the information to be sent, a section of fiber, often single-mode, carrying the signal emitted by the transmitting terminal, and a receiving terminal which serves to receive the optical signal transmitted by the fiber.
The performance of an optical transmission system, particularly in terms of signal quality and data rate, is limited by the optical properties of the optical fiber which is a source of physical phenomena that degrade optical signals.
Attenuation of optical power and chromatic dispersion were first apparent as the most limiting of the phenomena identified, and means for at least partially resolving the degradations that they cause have been proposed.
Another unfavorable phenomenon is polarization mode dispersion (PMD) (also referred to herein as “modal dispersion of polarization”). Such a phenomenon is no longer insignificant, because of the ongoing attempts to further increase the transmission rate of optical signals.
This phenomenon of polarization mode dispersion is particularly difficult to compensate for when the optical transmission system that it affects is a WDM (Wavelength Division Multiplexing) optical transmission system used for long-haul terrestrial links and operating at bit rates per channel of 10 Gbit/s and above. Each channel is affected differently by polarization mode dispersion, requiring the use of single-channel compensation solutions.
With such data rates, the influence of polarization mode dispersion on fibers becomes problematic. A significant number of fibers in the networks of established carriers have PMD characteristics, so that transmission at 10 Gbit/s and beyond is not possible without changing all or part of the cable which represents a significant investment for the carriers concerned.
Certain optical devices for polarization compensation have been proposed for attempting to compensate for the impact of polarization mode dispersion but are still not in use at 10 Gbit/s, both because of their high cost and because these solutions are more suitable for single-channel systems (while WDM links contain several dozen channels) and have limited potential in terms of gain and response time. Most of these optical compensation devices require a feedback signal which necessitates a delay between detection and degradation and the application of the right parameters in the compensation device.
Other solutions for electronic compensation at 10 Gbit/s, based on FFE/DFE (Feed Forward Equalizer, Decision Feedback Equalizer) or MLSE (Maximum Likelihood Sequence Estimator) equalizers have also been developed and represent a possible alternative to cable replacement. These solutions, however, are still based on the concept of one compensator per channel, which represents a high cost if compensating for polarization dispersion in a WDM system.